1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus that includes the fixing device.
2. Description of the Related Art
A typical fixing device that fixes an unfixed toner image on a recording medium by applying pressure and heat thereto includes a heating unit and a pressurizing unit. The heating unit includes an elastic layer and a heat source and the pressurizing unit includes an endless member and a pressurizing member. The recoding medium with an unfixed toner image thereon is conveyed to a nip portion between the heating unit and the pressurizing unit so that the unfixed toner image is fixed to the recoding medium. Various types of such fixing devices that employ the belt-nip method have been proposed. In the belt-nip method, a nip portion is formed in a way that a pressurizing member is pressed against a heating roller that is rotatable and that includes an elastic layer with the surface elastically deformable. The pressurizing member locally deforms a portion of the heating roller on a side from where a recording medium exits the nip portion. However, such deformation tends to change the surface speed of the fixing roller, causing shift of an image on the recording medium. In addition, a hard member is pressed against the elastic layer of the heating roller to locally cause deformation of the elastic layer so that a nip portion having a small curvature is formed. Hence, a large load is applied to the recording medium, for example, an amount of damage on the recoding medium or curling of the recoding medium increases.
To deal with such inconvenience, Japanese Patent Application Laid-open No. 2004-045780 discloses a pressurizing unit including an entering pad on a side from which a recording medium enters a nip portion, an exit pressurizing roller on a side from which the recording medium exits the nip portion, and a center pad between the entering pad and the exit pressurizing roller. The entering pad, the center pad, and the exit pressurizing roller are pressurized individually. The center portion of the pressurizing unit has a lower surface so as to have a V-shaped portion.
Furthermore, Japanese Patent Application Laid-open No. H08-166734 discloses a technology for preventing an image on a recording medium from shifting. Near the exit of a nip portion, a pressurizing roller around which a pressurizing belt extends is pressed against a fixing roller with the surface on which an elastic layer is provided. Accordingly, the elastic layer deforms and the deformation helps a recording medium to be released from fixing roller. An auxiliary roller that includes an elastic layer is provided to an upstream portion in a direction of conveyance of the recording medium. The auxiliary roller is pressed against the fixing roller to prevent the recording medium from being conveyed at a speed higher than that of the surface speed of the fixing roller, thus preventing an image from shifting on the recording medium. The shifting of the image can be effectively prevented when the total of the pressure from the auxiliary roller and the pressure from a tensile force of the pressurizing belt is larger than that from the pressurizing roller.
In addition, Japanese Patent Application Laid-open No. H11-212389 discloses a heating-pressurizing fixing device that includes a heating roller, a pressurizing belt that is pressed against the heating roller, and a pressurizing roller that pressurizes the pressurizing belt. The heating-pressurizing fixing device fixes an unfixed toner image on a recording medium by heat and pressure while causing the recording medium to pass through a nip portion that is formed between and by the heating roller and the pressurizing belt. The heating-pressurizing fixing device further includes a pressurizing member that has a round edge.
Moreover, Japanese Patent Application Laid-open No. S63-036283 discloses a thermal fixing roller that includes a core around which silicon rubber is formed. The silicon rubber has hardness equal to 35 degrees (JIS A) or smaller, a permanent deformation equal to 10% or smaller, a thickness of 1.5 mm to 4 mm. Japanese Patent Application Laid-open No. 2002-207388 discloses an image forming unit that includes a pressurizing belt, a pressurizing-belt supporting member, and a pressuring unit. The pressurizing belt presses a recording medium against a heating-fixing roller. The pressurizing-belt supporting member includes a plurality of supporting members around which the pressurizing belt extends. The pressing unit presses the recording medium between the heating-fixing roller and the outer surface of the pressurizing belt. As the pressing unit, at least a pair of pressing members is provided such that one of the pressing members is positioned on the inner surface of the heating-fixing roller and the other pressing member is positioned on the inner surface of the pressurizing belt.
Furthermore, Japanese Patent Application Laid-open No. H09-251252 discloses a pressurizing member that has a shape in which the thickness of the pressurizing member in the vertical direction gradually decreases from each of the two edges to the center thereof (hereinafter, “inverted-crown shape”). The pressurizing member includes an elastic layer that has the two edge portions whose foam densities are different from that of the center portion of the elastic layer such that the hardness of the edge portions is higher than that of the center potion.
Moreover, Japanese Patent Application Laid-open No. H03-233586 discloses a fixing device that includes a plurality of rollers, an endless belt that is wound around the heating member and the rollers, a heating member, and a pressurizing roller. The heating member has a shape in which the thickness of the heating member in the vertical direction gradually increases from each of the two edges to the center thereof (hereinafter, “crown shape”), and the pressurizing roller has the inverted-crown shape and is pressed against the heating member.
In addition, Japanese Patent Application Laid-open No. H2001-296691 discloses a heating roller that has a diameter equal to 28 mm or smaller. The heating roller includes a rubber layer that has a thickness of 0.5 mm to 10 mm, a hardness of 8 (JIS-A) or smaller. Japanese Patent Application Laid-open No. 2002-25713 discloses a roller that includes an elastic layer that has a hardness of 15 degrees to 55 degrees (Asker-C) and a permanent deformation (compression) of 0.5% to 5%.